The invention concerns a programmable hybrid hearing aid with digital signal processing and a method for detection and signal processing in a programmable hybrid hearing aid.
Present day hearing aids are usually based on analog amplification of the sound intercepted by the ear. With the aid of present day state of the art, hearing aids of this kind have become miniaturized to such an extent that they can be inserted into the outer meatus, thus constituting so-called "all-in-the-ear" aids. Many people prefer hearing aids of this type for reasons of appearance and comfort, but the use of analog amplification of the sound signal combined with the fact that these hearing aids close off the meatus, make it difficult to obtain an optimum adaptation of the signal to any hearing residue which the person using the hearing aid may still have. Most forms of age-dependent hearing impairment leave a substantial amount of hearing residue in certain frequency ranges. In the case of normal neurologically-dependent hearing impairment the sense of hearing usually remains relatively unimpaired at the lowest frequencies. If the ear is completely closed by the hearing aid, the sound has to be amplified at all frequencies in the audible range. At the same time, the use of ordinary analog amplification makes it difficult to obtain an optimum response function, i.e. a response function which in an appropriate manner simulates the acoustic response of the meatus when it is open without insertion amplification. Any hearing residue which the user may have will result in the amplification in an all-pass band giving rise to discomfort, e.g. if impulse noise and transient acoustic signals are amplified in those frequency bands where the ear still has a reasonably normal degree of hearing. Moreover, an open meatus normally has a resonance of approximately 3 kHz, and this resonance makes a vital contribution to the quality of the auditory impression, since it falls within the range of the formant frequency for normal speech and thus contributes to giving it its tonal qualities, which are tremendously important for the comprehension of speech sound and thus for the person's ability to understand speech.
In order to facilitate the optimum adaptation of the auditory signal to any hearing residue and simultaneously optimize the hearing aid's response function, hearing aids have been developed wherein the signal processing is performed digitally. The response function is adapted through filtering of the digital signal by means of appropriate filter coefficients, thus permitting the frequency response to some extent to simulate the response function of a person with normal hearing. If the aids of the digital type are designed as so-called all-in-the-ear aids, the problem again arises that the meatus is closed, thus preventing any hearing residue which the person may have from being utilized. The response curve can be modified to a certain extent in order to take this into consideration. As a rule, however, it will be an advantage to have several response curves, in order to adapt the hearing aid's amplification as a function of the frequency to a variety of acoustic environments. It is obvious, e.g., that it would be considerably more difficult to understand normal speech which is embedded in loud background noise, in which case it will be natural to generate a response function which gives priority to amplification in the range of the speech signal's formant frequencies, i.e. primarily in the range from approximately 1 up to approximately 4 kHz.
Another well-known problem with hearing aids, whether they are digital or analog, is acoustic feedback between sound generator and microphone. Even though the hearing aid is positioned so that it closes the meatus and thus also prevents utilization of any hearing residue, this does not prevent feedback at high amplification, since the sound from the sound generator can be conducted back to the microphone either via the material of the hearing aid or via tissue and bone matter in the vicinity of the meatus. It will therefore be desirable to cancel such a feedback signal, e.g. in connection with the digital signal processing in the hearing aid. As has already been mentioned it is also desirable to utilize any hearing residue at lower frequencies, and this requires the meatus to be at least partially open, preferably so that it creates an acoustic transmission channel with a low-pass characteristic between the ear opening and the tympanum. If a channel of this kind is to be used with a hearing aid of the all-in-the-ear type, this makes great demands on the miniaturization of the hearing aid. Moreover, the problem of acoustic feedback will be further accentuated and will need to be eliminated in one way or another.
Digital hearing aids of the above-mentioned kind are known from, e.g., U.S. Pat. No. 4,471,171 (Kopke et al.), where a digital data processor for processing of digitalized audio signals is connected to a programmable memory which stores predetermined response functions in accordance with the user's requirements or preferences and/or the use of the hearing aid, so that the use of the hearing aid can be directly adapted to the requirements of the user, while at the same time it is possible to program the hearing aid in step with any alterations in the user's hearing ability or response characteristics.
Similarly, U.S. Pat. No. 4,731,850 (Levit et al.) contains a programmable hearing aid with digital filters where coefficients are supplied from a programmable read-only memory to a programmable filter and an amplitude limiter in the hearing aid, enabling this to be automatically adjusted to an optimum set of parameter values for speech level, echo and type of background noise while simultaneously facilitating a reduction of acoustic feedback, in that an electrical feedback path in the aid is adapted to the acoustic feedback path both in amplitude and phase, causing the two feedback signals to be cancelled by subtraction. GB-PS no. 1 582 821 principally contains a hearing aid for digital signal processing by means of a programmable memory which can be fed with values taken from an audiometrically determined audiogram.
The above-mentioned U.S. Pat. No. 4,731,850 also contains a hearing aid which uses one or more microphones, so that the weighted, summed output signal from the microphones with a suitable phase displacement is equal to the output signal from a frequency selective, directive microphone. This should be able to reduce the effect of both noise and echo. Furthermore, cancellation or suppression of acoustic feedback in hearing aids is discussed in the article "Measurement and Adaptive Suppression of Acoustic Feedback in Hearing Aids" (Bustamante et al.), IEEE Transactions on Acoustics, Speech and Signal Processing, 1989, No. 2, pp. 2017-20. The authors discuss three methods for suppressing acoustic feedback, viz. time-variable delay, adaptive inverse filtering and adaptive feedback cancellation, and find that the latter method is the most successful, since it increases the maximum amplification in the hearing aid by 6-10 dB without acoustic feedback.
It should also be mentioned that there are known hearing aids of the all-in-the-ear type where there is an open connection between the ear opening and that portion of the inner meatus which is situated close to the tympanum. The object of this known, open connection is to obtain an equalization of pressure variations in the outer meatus adjacent to the tympanum.
None of the above-mentioned constructions or methods, however, provides any directions as to how to achieve a hearing aid, preferably of the all-in-the-ear type, which simultaneously offers the possibility of utilizing a user's low frequency hearing residue, while at the same time generating a response curve which gives an optimum simulation of the meatus's natural response function in the frequency range which is required in order to reproduce high quality speech sound.